1. Field of the Invention
The present invention relates to a liquid crystal display device applied to a liquid crystal display panel of, for example, optically compensated bend (OCB) mode.
2. Description of the Related Art
A flat-panel display device represented by a liquid crystal display device is widely utilized as display device for a computer, a car navigation system, a television receiver, etc.
In general, the liquid crystal display device comprises a liquid crystal display panel that includes a matrix array of liquid crystal pixels, and a display control circuit that controls the display panel. The liquid crystal display panel has a structure in which a liquid crystal layer is held between an array substrate and an counter substrate.
The array substrate includes a plurality of pixel electrodes arrayed substantially in a matrix, a plurality of gate lines arranged along the rows of pixel electrodes, a plurality of source lines arranged along the columns of pixel electrodes, and a plurality of switching elements arranged near intersections between the gate lines and the source lines. Each switching element is made, for example, of a thin film transistor (TFT), and is turned on to apply the potential of a corresponding source line to a corresponding pixel electrode when a corresponding gate line has been driven. On the counter substrate, a common electrode is provided to face the pixel electrodes arrayed on the array substrate. Each pair of the pixel electrodes and common electrode serves as a pixel together with a pixel region of the liquid crystal layer, and controls the alignment of liquid crystal molecules in the pixel region by an electric field between the pixel electrode and the common electrode. The display control circuit includes a gate driver for driving the gate lines, a source driver for driving the source lines, and a controller for controlling operation timings of the gate driver and source driver.
In the case where the liquid crystal display device is used for a television receiver that principally displays a moving image, a liquid crystal display panel of an OCB mode, in which liquid crystal molecules exhibit a good response characteristic, has begun to be employed (refer to Jpn. Pat. Appln. KOKAI Publication No. 2002-202491). In this liquid crystal display panel, liquid crystal molecules are set to a splay alignment before supply of power by alignment layers which have been rubbed parallel to each other on the pixel electrode and the common electrode. The liquid crystal display panel begins a display operation after the liquid crystal molecules have been transferred from the splay alignment to a bend alignment by a relatively strong electric field applied in an initialization process which is performed upon supply of power.
A reason why the liquid crystal molecules get in the splay alignment before supply of power is that the splay alignment is more stable than the bend alignment in terms of energy in a no-voltage-applied state of a liquid crystal drive voltage. Even after the liquid crystal molecules have been transferred to the bend alignment, the bend alignment of the molecules tends to be inverse-transferred to the splay alignment if a no-voltage-applied state or a voltage-applied state of a voltage not higher than a level at which the energy of splay alignment is balanced with the energy of bend alignment, continues for a long time. The viewing angle characteristic of the splay alignment significantly differs from that of the bend alignment. Thus, a normal display is not attained in the splay alignment.
In a conventional driving method that prevents the inverse-transfer from the bend alignment to the splay alignment, a high voltage is applied to liquid crystal molecules in a part of one frame period for display of single-frame image, for example. This high voltage is equivalent to a pixel voltage for a black display in a liquid crystal display panel, which is a normally-white type, so this driving method is called “black insertion driving”.
Meanwhile, in a case where a backlight is used as an illumination light source in a display operation of the liquid crystal display panel, an illumination request is supplied from a video signal source, which is a user circuit for the liquid crystal display panel. If the backlight is driven immediately after supply of power, an unstable image appears on the liquid crystal display panel because of the liquid crystal molecules incompletely transferred from the splay alignment to the bend alignment. The transfer time required for reliable transfer from the splay alignment to the bend alignment depends on an ambient temperature around the liquid display panel. As a technique of keeping the unstable image out of view, the backlight may be controlled to illuminate the display panel at a timing sufficiently delayed after supply of power. However, this technique raises the problem that the backlight would not turn on quickly, even if transfer from the splay alignment to the bend alignment has been completed in a short period of time and the liquid crystal display panel has become a state capable of displaying a stable image corresponding to the video signal.